Ultrasonic imaging techniques and mammograph equipment

ABSTRACT

An improved ultrasonic imaging system specially adapted for examination of a female breast utilizing direct imaging (twodimensional) or holographic imaging (three-dimensional) techniques. Improvements in both types of ultrasonic imaging are described which utilize an improved fluid-filled lens for imaging an ultrasonic field passing through the breast or other object under investigation onto an area detector from which object information may be read out by light. Use of an ultrasonic lens in the reference beam of a holographic imaging system is also disclosed. A technique for holographic imaging wherein viewing optics are focused on the area detector directly instead of upon a focused image in space is also described with improved results under certain circumstances.

United States Patent Inventor Byron B. Brenden Richland, Wash.

Appl. No. 730360 Filed May 20, 1968 Patented June 22, 1971 AssigneeHolotron Corporation Wilmington. Del.

Continuation-impart of application Ser. No. 710,991, Mar. 6, 1968, nowabandoned.

ULTRASONIC IMAGING TECHNIQUES AND MAMMOGRAPHEQUIPMENT 18 Claims, 28Drawing Figs.

US. Cl. 73/676, 340/5, 350l3.5 Int. Cl 601:: 29/04 Field of Search73/675 H; 350/35; 340/5 H References Cited UNITED STATES PATENTS 9/l968Silverman OTHER REFERENCES Leith et al., Holograms: Their Properties andUses," S.P.I.E. JOURNAL Oct./Nov. I965, p. 3- 6.

Tanner, L. H. Some Applications of Holography in Fluid Mechanics,"JOURNAL OF SCIENTIFIC INSTRUMENTS, V. 43, No. 2, Feb. 1966, p. 8l 83.

Primary Examiner-Richard C. Queisser Assistant Examiner-John P.Beauchamp Attorney-Woodcock, Phelan & Washburn ABSTRACT: An improvedultrasonic imaging system specially adapted for examination of a femalebreast utilizing direct imaging (two-dimensional) or holographic imaging(three-dimensional) techniques. Improvements in both types of ultrasonicimaging are described which utilize an improved fluid-filled lens forimaging an ultrasonic field passing through the breast or other objectunder investigation onto an area detector from which object informationmay be read out by light. Use of an ultrasonic lens in the referencebeam of a holographic imaging system is also disclosed. A technique forholographic imaging wherein viewing optics are focused on the areadetector directly instead of upon a focused image in space is alsodescribed with improved results under certain circumstances.

PATENTEUJUNZZIHYI 3.585847 SHEET 03 0F 11 PATENIED JUN22 can SHEET 05 OFPRIOR ART Fl' /6A PATENTEU JUN22 |97i SHEET 08 0F PATE -NTED June 219msum as 0F ULTRASONIC IMAGING TECHNIQUES AND MAMMOGRAPI'I EQUIPMENTCROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of application Ser. No. 710,99l, filed Mar. 6,1968, by Byron B. Brenden, now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally toultrasonic imaging systems and more. specifically to improvements in themethod and apparatus used in the art of ultrasonic imaging.

Many testing techniques involving ultrasonic techniques are beingused.'Ultrasonic imaging utilizing pulse-echo techniques fortwo-dimensional imaging of an object under inspection is one well-knowntechnique. The time delay and intensity of the reflection of successiveultrasonic energy pulses scanned across an object are put together tomap out the internal structure of an object. This technique is morefully described elsewhere, such as by Carlin, Physical Acoustics, Vol.I, Part B, page 52, edited by Mason (1964).

Direct ultrasonic imaging in two dimensions also has been applied tomaterials testing. A beam of ultrasonic energy is passed through anobject under investigation and then to an area detector which isilluminated with light to render an image of the transparency of theobject to ultrasound. If there is some internal defect in the object,its image will be projected onto the area detector and thus made visiblein two dimensions. If an ultrasonic lens is placed between the objectand area detector to image the ultrasonic field passing through theobject onto the area detector, a better image is obtained of the flaw.One example of this technique is described by Hueter and Bolt in Sonics,page 353, published by Wiley in 1955.

For certain object inspections, it is desirable to be able to inspectthe internal structure of an object as seen in three dimensions byultrasound passing through it. This result is obtained by recentimprovements in the techniques of ultrasonic imaging which make use ofthe phenomenon of wave front reconstruction or holography. In apreferred form of ultrasonic holography, a standing wave patterncomprising a hologram is formed at an area detector in a fluid medium bythe interference created between two ultrasonic beams, each atsubstantially the same ultrasonic frequency (i.e., the ultrasonic beamsare mutually coherent so that an interference pattern is formed at thearea detector), and directed to the area detector at a finite angletherebetween. One of the ultrasonic beams is caused to pass through anobject under inspection and therefore its wave front containsinformation of the object and any internal flaws or defects therein.This information is transferred to the standing wave pattern byintroducing the second ultrasonic beam (reference beam) to interferewith the object beam, somewhat analogous to the interference in lightholography. The standing wave pattern (ultrasonic hologram) can thenreflect light into various diffracted orders and an image, either actualor conjugate, of the original object can be viewed by positioningsuitable viewing optics focused on the desired image in either of thetwo first order diffracted beams, respectively.

The principles of ultrasonic holography are described and claimed incopending patent application Ser. No. 569,914, filed Aug. 3, 1966. Animproved method of ultrasonic holography in which objects can be viewedin different colors according to variations in the density of an objectis described and claimed in copending patent application Ser. No.691,253, filed-Dec.- I8, 1967.

It is an object of this invention to present an improved technique forviewing images of improved quality from ultrasonic holograms. I

It is a further object of this invention to provide a method andapparatus for producing an ultrasonic hologram with reduced noise.

SUMMARY OF THE INVENTION A three-dimensional ultrasonic holographicsystem wherein an object beam and reference beam interfere at an areadetector to produce an ultrasonic hologram includes focusing viewingoptics directly on the hologram instead of on a. focused image in space,thereby allowing the use of a substantially larger point source of lightfor illuminating the hologram, eliminating the image degradation causedby deviations in the hologram surface from that of a plane and allowingeasier viewing of an image in a color ultrasonic holography system. Anydegradation of the resolution of the image so viewed because the viewingoptics are not focused directly upon the image may be overcome by theuse of an ultrasonic lens placed to image the ultrasonic field passingthrough the object onto the area detector.

Another aspect of the three-dimensional ultrasonic hologram imagingsystem involves placing an ultrasonic lens in the reference beam in aposition to image a transducer producing the reference beam onto thearea detector, thereby to make a hologram with less extraneous noise. Tofurther reduce noise in the hologram, a pinhole filter may be placedbetween the lens and the area detector to result in an improvedspherical wave front. This improvement also makes the choice of atransducer less critical.

While the scope of the invention is defined in the appended claims, thisinvention may be best understood by reference to the followingdescription of the preferred embodiments taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 illustrate a method oftwodimensional ultrasonic direct imaging;

FIG. 3 shows apparatus useful in carrying out ordinary ultrasonicholographic image reconstruction and which is also useful in carryingout certain improved techniques of the present invention;

FIG. 4 illustrates an improved holographic imaging technique of thisinvention;

FIG. 5 illustrates another embodiment of an improved holo' graphicimaging technique which also utilizes an ultrasonic lens in the objectbeam;

FIGS. 6 and 6A illustrate an improved holographic imaging system when aspatial filtering system is utilized in the optical domain;

FIG. 7 illustrates the problem of edge effects from a quartz transducerin the reference beam;

FIG. 8 shows the use of an ultrasonic lens for imaging a reference beamtransducer onto an area detector for making a hologram;

FIG. 9 illustrates the use of an ultrasonic lens and pinhole filter toproduce an improved reference beam for use in ultrasonic holography;

FIG. 10 shows the use of an ultrasonic lens and pinhole filter with aspherical-shaped transducer to produce an improved reference beam foruse in ultrasonic holography;

FIG. 11 illustrates in cross section a preferred ultrasonic lens;

FIG. 12 demonstrates a problem in maintaining quality of an ultrasoniclens according to FIG. 11;

FIGS. 13, 13A and 13B illustrate a method for making an improvedultrasonic lens according to this invention;

FIGS. 14 and 14A show the construction of an improved ultrasonic lensaccording to this invention in a preferred embodiment;

FIG. 15 shows apparatus for controlling the quantity of fluid within theultrasonic lens of FIGS. 14 and 14A FIG. 16 illustrates in cross sectionan ultrasonic lens according to another embodiment of the presentinvention;

FIG. 16A illustrates in cross section an ultrasonic lens according to afurther embodiment of the present invention;

FIG. 17 illustrates mammograph equipment utilizing direct ultrasonicimaging techniques; I

FIG. 18 is an expanded view of the breast holder utilized in FIG. 17;

FIG. 19 shows ultrasonic mammograph equipment which utilizes thetechniques of holographic imaging;

FIG. 20 is a top view of FIG. 19 taken at section 20-20;

FIG. 21 is an end view of FIG. 19 taken at section 21-21 whichadditionally illustrates schematically an optical system for reading outa'three-dimensional image;

FIG. 22 shows a preferred patient support plate which may be used in theultrasonic mammograph apparatus of FIGS. 17 and 19; and

FIG. 23 is a breast holder which is preferred for use in the ultrasonicmammograph equipment of FIGS. 17 and 19.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the broadest sense, thesonic frequencies utilized in ultrasonic imaging are not limited to anyparticular range but include the entire spectrum of compressional waveenergy. However, in the more practical embodiments of that technique, ithas been found that the higher sonic frequencies (i.e., thoseconsiderably above the audible range) are much more desirable than thelower sonic frequencies. For this reason, instead of utilizing the moregeneral term "compressional wave energy," the term ultrasonic energy"will be utilized in the following description. This should, however, inno way limit the scope of the invention.

Furthermore, the medium in which such ultrasonic energy is propagated isreferred to in this description as a liquid, since materials in thisclass are preferred. However, this should not limit the scope of theinvention, since any ultrasonic transmitting medium may be utilizedwhich has the physical property which best serves the purpose of theparticular embodiment.

Visualization of ultrasonic wave fronts or fields passing throughobjects has previously been employed to study the properties of suchobjects and to determine the existence of flaws or defects. Inapplications where the imaging of the internal structure of objects isaccomplished by the visualization of ultrasonic fields, the technique issimilar to an X-ray technique, although the physical interactions of theultrasound with the object is much different than the interaction ofX-rays. Referring to FIG. 1, there is shown in diagrammatical form onearrangement utilized to image an internal flaw in a metal plate.Ultrasonic energy generated by an ultrasonic transducer 12 positionedwithin a liquid-filled tank 13 is transmitted through a metal plate 14to the surface 15 of the liquid. A void or flaw 16 within the metalplate 14 is opaque to ultrasound and therefore modifies the ultrasonicbeam transmitted from the ultrasonic transducer 12. At the surface 15the ultrasonic energy creates a distortion pattern proportional to theintensity of the ultrasound and this distortion pattern constitutes animage of the interior of the metal plate, including the flaw 16. Thisultrasonic image can be rendered in visible light by illuminating thesurface 15 from a point source of light 17 and forming an image at plane18 of that portion of the surface 15 illuminated by means ofa lens 19.Light which is reflected from perfectly horizontal portions of thesurface of the liquid and contains little desired information of theflaw 16, is focused by the lens 19 and blocked by a filter 20, therebyimproving the clarity of the image.

The intensity of the light in the image plane 18 does not correspond tothe intensity of the ultrasound at the surface 15, but rather isproportional to the rate of change of the ultrasonic intensity at thesurface 15. Therefore, the arrangement of FIG. 1 tends to show only theedges of the flaw 16 as illustrated in FIG. 2, which is a representationof the image plane 18 turned 90. An image 16' illustrated in FIG. 2 isan outline of the flaw 16 rather than a true image of the same.Therefore, in the method illustrated in FIGS. 1 and 2, the imageobtained from the utilization of a single ultrasonic beam and a liquidsurface area detector does not contain a light intensity distributionwhich is proportional to the ultrasonic intensity distribution at theliquid surface.

There are several ultrasonic area detectors known in the art of directultrasonic imaging which will in certain circumstances produce anoptical image which is a more faithful representation of the ultrasonicwave front passing through the object than the liquid surface areadetector shown in FIG. I. For instance, one area detector is known as aPohlman cell wherein specular reflecting flakes are suspended in aliquid contained between two windows. The cell is placed in the path ofthe ultrasonic beam after it passes through an object. The reflectiveflakes, often made of aluminum, are left free to orient themselvesaccording to the direction of the ultrasonic wave front passing throughthe detector, thereby to give an optical representation of the travelingultrasonic wave front. This detector is described basically by R.Pohlman in Z. Physik, 1 13,697 (1939), in an article entitled On thePossibility of an Acoustic Image in Analogy to an Optical One.

Another direct imaging area detector is the ultrasonic camera whichutilizes a quartz transducer in the path of the ultrasonic beam afterpassing through the object. The transducer is scanned by a electronicbeam in a manner similar to that of a television picture tube. Theelectronic beam is modulated in intensity according to the charge on thetransducer which in turn corresponds to the characteristics of theultrasonic wave front striking the transducer. A television displaymonitor may then be used to display an optical representation of theultrasonic wave front striking the transducer. More details of this typeof detector may be had by reference to an article entitled UltrasonicImage Camera," Engineer, 207,348 1959).

A third method of detecting ultrasound in a defined area is to scan thearea with a substantially point ultrasonic sensitive transducer. Theultrasonic field may then be reconstructed in the optical domainaccording to the scanning pattern. This technique is disclosed morefully by Preston and Kruezer, Applied Physics Letters 10,5, l50l52I967).

The principles of ultrasonic holography will be described with referenceto FIG. 3. A liquid-filled tank 22 contains two ultrasonic transducers24 and 26 which direct ultrasonic beams 28 and 30 of substantially thesame frequency to a liquid surface 32. An object 34 containing, forexample, a flaw 36, is placed in one of the beams 30 (object beam) andthe other beam 28 acts as a reference beam to interact with the object30 at the surface 32 to form an interference standing wave pattern 31. Apoint source of light radiation 38 illuminates the interference patternor ultrasonic hologram formed at the liquid surface 32 and the hologramdiffracts the illuminated light into various diffracted orders,including a zero order and two first orders, which are gathered by alens 40 and focused to spatially displaced focal points at a spatialfilter 42. The spatial filter 42 blocks all undesired diffracted ordersof light and allows only one desired first order beam to pass. Theunblocked first order beam, which in FIG. 3 is shown to be the +1 firstorder beam, contains either an actual or conjugate image of the object34 and the flaw 36, and this image can be viewed by focusing anobservers eye 45, aided by a suitable eyepiece 44, on this image. Theimage viewed is a replica of the object beam wave front as it passedthrough the object 34, transformed from an ultrasonic to an opticaldomain.

Holographic imaging preferably utilizes an area detector which willdetect a standing wave resulting from interference of two ultrasonicenergy beams for diffracting light incident thereon into its variousdiffracted orders. This is to be distinguished from the direct imagingsystems wherein an area detector is called upon to give directly avisual indication of the traveling ultrasonic field striking it. For aholographic system, a liquid interface such as shown in FIG. 3 ispreferred for producing a standing wave pattern and the best resultshave been found by using an isolation tank as described hereinafter andin copending applications Ser. No. 6I3,5ll filed Feb. 2, 1967, nowabandoned; Ser. No. 613,7ll filed Feb. 8, I967, now patent No.3,419,934; and Ser. No. 710,893 filed Mar. 6, 1968. A liquid interfaceisolation tank is inexpensive for a wide detecting area and allows bothviewing an image in real time and the making of a permanent hologram onphotographic film.

It should be noted that the term "viewing optics as used throughout thisdescription refers to the entire optical system utilized to control thelight after being diffracted by the hologram. ln HO. 3, this includesthe lens 40, the eyepiece 44 and the eye 45. This is only one example ofviewing optics which may be used in this invention. As an alternative,the eyepiece 44 and the eye 45 may be replaced by a photographic camera,a television camera, or other optical means.

As a part of the present invention improving ultrasonic holography, ithas been discovered that the quality (resolution) of an imagereconstructed and viewed directly according to the prior techniques ofultrasonic holography are affected by the size of the point light source38 and any irregularities in the surface of the area detector 31 of FIG.3. lf a perfect point source of light is used and if the area detectoris a perfect plane, the best image is obtained byfocusing the viewingoptics on aplane passing through an image. However, in actual practice,point" light sources have some finite size and a liquid area detectorhas definite finite imperfections. it has been discovered that as theviewing optics are focused on a plane in space which becomes closer tothe hologram surface and further from the position of the focused objectimage, an image representation will be viewed whose resolution is lessadversely affected by a finite light source 38 and an irregular areadetector surface 31. When the viewing optics are focused on the surface31, a light image of the object 34 is obtained which is independent ofthese two image degradation factors and thus has improved resolution.Therefore, a larger light source 38 may be used to result in a brighterviewed image and further, extraordinary techniques for compensating foran irregular surface 31 need not be taken, all without affecting thequality of the viewed image of the object. Resolution of the imageviewed is, of course, reduced somewhat when the viewing optics arefocused somewhere in space other than on a plane passing through thefocused image to be viewed. It has been found that resolution loss maybe regained by use of an ultrasonic lens in the object beam, as will beexplained more fully hereinafter.

To understand more fully the operation of this technique, referenceshould be made to FlG. 4 wherein an ultrasonic hologram of an object 54having a point P and immersed in a liquid 55 is being made by object andreference ultrasonic beams (not shown). The interfering object andreference ultrasonic beams form a standing wave pattern (ultrasonichologram) 56 at the liquid surface area detector 57. A point lightsource 58 at the focal point of a lens 59 produces a collimated lightbeam 60 for illuminating the ultrasonic hologram 56. This incident lightis diffracted into a +1 diffracted order 61 and a -l diffracted order62. A zero order beam 63 is a reflection of the incident light beam 60from the ultrasonic hologram 56 without diffraction. The zero order beam63 contains object information similar to that contained in thereflected light of FIG. 1 that is blocked by the filter 20. it isusually necessary to separate these diffracted orders from each otherand from the zero order beam by the use of a lens 40 and spatial filter42 as shown in FIG. 3, but for clarity of explanation, it is assumedthat the image information in each of the diffracted orders of FIG. 4may be viewed without interference from the other diffracted order or bythe zero order beam. images P and P" are formed in the diffracted firstorders of light of the point P of the object 54 and may be viewed bysuitable viewing optics such as an eyepiece 64 and an eye 66 focusedupon an actual image P (or an eyepiece 65 and an eye 67 focused upon aconjugate image P") in accordance with the techniques, of ultrasonicholography before the present invention. I By the technique of thisinvention, the viewing optics are focused on the hologram 56, therebyproviding an object image of improved resolution, as hereinabovedescribed.

The loss of resolution by focusing the viewing optics upon the hologram56 instead of directly upon the point P has been found not to be sogreat as to lose all value of the hologram surface focusing technique.The point P of the focused image is located a distance from the hologram56 which is related directly to the distance that the point P on theobject 54 is placed away from the hologram 56, and is also directlyrelated to the ratio of the ultrasonic wavelength to the reconstructinglight wavelength. Therefore, if the point P is moved closer to thehologram surface 56, the position of the focused image P will becomecloser to the hologram 56, resulting in an optical image of increasingresolution when the viewing optics are focused on this surface. Theposition of the object 54, however, is limited in that it cannot beplaced so close to the hologram surface 56 that it gets in the way ofthe reference ultrasonic beam (not shown in FIG. 4). One technique forrearranging the object and reference ultrasonic transducers to allow theobject to be placed closer to the hologram surface is disclosed in theaforementioned copending Pat. application Ser. No. 710,893.

An alternative to placing the object near the hologram detecting surfaceis to interpose an ultrasonic lens between the object and this surfacefor imaging the ultrasonic field passing through the object onto thehologram detecting surface. Apparatus for this is shown in FIG. 5. Theresult of using such a lens, referring again to F IG. 4, is to place thefocused image P at the hologram surface 56, the plane at which theviewing optics are focused. 5

Referring now to FIG. 5, an ultrasonic lens 50 is placed in the path ofthe object beam 30 and functions to place an ultrasonic image of theobject 34 with its flaw 36 directly into the plane of the liquid surface32. lmaging of the flaw 36 by the ultrasonic lens 50 into the ultrasonichologram at the surface 32 is indicated by the dotted lines 52.Depending on the quality of the ultrasonic lens 50, the ultrasonic wavefront existing at the object is reproduced at the hologram surface, sothat a portion of the three-dimensional image reconstructed by incidentlight appears to be in the hologram plane. Although the flaw image isshown to be in the plane of the surface 32, it may be that this imagewill lie in some other plane at an angle to the surface. An ultrasoniclens is made of a material in a configuration to bend incident sonicbeams much in the same way that an optical lens refracts incident light.More information concerning ultrasonic lenses may be had by reference tothe aforementioned textbook Sonics by Hueter and Bolt, published by JohnWiley 8L Sons in 1955, especially to the discussion on pages 265 and353. A lens structure having improved imaging characteristics over thatof the lenses described therein is described hereinafter.

.The aspect of the present invention wherein the viewing optics arefocused upon the hologram surface has a further advantage in the area ofcolor rendition ultrasonic holography as described and claimed in theaforementioned patent application Ser. No. 691,253. Briefly describingthe technique of color rendition from ultrasonic holograms, theultrasonic transducers utilized to produce the holograms aresuccessively driven at mutually exclusive ultrasonic frequencies,thereby producing successive ultrasonic holograms at thehologramdetecting surface. Each successive hologram can render an imagein a different colored light such that a plurality of different coloredimages are produced in space. By bringing these different colored imagesinto registration with appropriate magnification to make them all thesame size, a composite multicoloredimage may be viewed. A common way toaccomplish image registration is by means of lens segments provided inthe viewing optics to appropriately magnify and bend the light rays ofeach image to common points in space. This technique of imageregistration often imposes certain technical problems. However, byfocusing'the viewing optics on the hologram surface according to thepresent invention, this difficult optical image registration is avoided,since the various images appear in registration to form a multicoloredimage.

Since a focusing lens is generally utilized to spatially separate thevarious diffracted orders of light from an ultrasonic hologram, such asaccomplished by the lens 40 of FIG. 3, it is useful to consider aspecific example wherein this aspect of the present invention may applyby reference to FIGS. 6 and 6A. A lens 160 with a focal length F isplaced a distance b from the hologram surface 166 and in the path of thediffracted and zero order beams to bring each to a point focus at aspatial filter 162. An object 164 having a point P is placed a distancebelow the hologram surface 166. If we let u be the distance from thelens 160 to a desired image, A the ultrasonic energy wavelength and Athe light wavelength, we have the expression based upon the well-knownlens formula as follows:

where the plus and minus signs correspond to the location of the actualimage (FIG. 6) and the conjugate image (FIG. 6A), respectively. Theposition of an image of the hologram surface 166 is given by equation lwhere a=0.

As a specific example, if the lens 160 is placed 0.] meter from thehologram 166 and if a point P of an object 164 is placed 0.1 meter fromthe hologram surface 166 and the lens 160 has a focal length of 5 metersand the ratio of ultrasound wavelength to that of the reconstructinglight beam 168 is 200, equation (I) will give us the positions ofdesired images. An image 170 of the hologram surface '166 produced bythe lens 160 is calculated to lie 0.102 meters from the lens 160, asshown in both FIGS. 6 and 6A. An actual image P of the point P of object164 is formed by the lens 160 in the +1 first order diffraction beam adistance of 6.67 meters behind the lens 160, as shown in FIG. 6. Aconjugate image P" of point P of the object 164 is calculated to beformed 4.0 meters behind the lens 160 in the -l first order diffractedbeam, as shown in FIG. 6A. According to this invention, an eyepiece 172and an eye 174, or an eyepiece 176 and an eye 178, should be focused onthe hologram surface image 170 rendered by the lens 160, instead of on afocused image P or P". As point P is moved toward the hologram surface166 as a limit, the images P and P" will move toward the hologramsurface image plane 170 as a limit, thereby resulting in no loss ofimage resolution in this limiting case when the eyepiece (172 or 176)and eye (174 or 178) are focused on an image of the hologram surface.Point P of the object 164 may be so moved toward the hologram surface166 "either physically, if possible, or by the use of an ultrasoniclens, as hereinbefore described.

The description with respect to FIGS. 6 and 6A has broken the opticalsystem into its elements. Looking to the optical system as a whole, aswas done earlier in this description, the viewing optics" (including thelens 160, an eyepiece and an eye) are focused upon the hologram surface166 in carrying out this invention.

According to another aspect of this invention, a further improvement inthe techniques of three-dimensional holographic imaging has beendiscovered. As is well known, the reconstructing illuminating wave frontmust bear a faithful relationship to the ultrasonic reference beam wavefront. This is necessary to obtain an image which is a faithfulreproduction of the object as seen by ultrasound. It is easier to obtainthis correspondence with a regular spherical or plane wave frontreference beam.

In a flat, quartz transducer utilized to produce a reference beam, ashas been illustrated hereinbefore, an irregular wave front is thought tobe caused by undesired wave elements emitted from the transducer edges.Referring to FIG. 7, a desired plane wave front reference beam 80 isemitted by a flat quartz transducer 82 which has a round area ofdiameter emits a nonplanar wave front 88 which spreads with distancefrom the transducer with an effective spread angle of 2B but the exactcharacter of this undesired wave front is not critical in order tocorrect for it.

If the transducer 82 is placed a distance y from the area detector 86which is short enough so that the cone-shaped wave front 88 will have aneffect over a very small area, a large area will be left that isaffected only by the desired plane wave 80. However, this is notpractical for almost all ultrasonic holography applications where thedistance y must be so great that the conical beam wave front 88 willcover the entire area desired to be used as the area detector. It is inintermediate areas where this undesired effect is at its worst and isthought to be the result of interference at the area detector betweenthe plane wave and the conical beam wave front 88. However, it has beenfound that if the distance y is given by the following expression,

the undesired interference effect is not too objectionable since theangle between the interfering beams 80 and 88 at the surface 86 becomessmall and the transducer looks like a point source. Also, the intensityof the conical beam 88 drops off faster with increased distance thandoes the plane wave 80. As an example, if the transducer 82 has adiameter D equal to 2 inches and the ultrasonic frequency being emittedis about 3 MHz., thus having a wavelength A in water of about 0.02 inch,the quantity y is calculated by equation (2) to be 50 inches. Therefore,for such a configuration, the transducer 82 must be placed at least 50inches from the liquid surface 86 to minimize the adverse effect of theconical wave front 88, too far to be convenient for most applications.

If the ultrasonic wave front leaving the transducer can be imaged ontoor near the liquid surface 86, the result will be substantially the sameas if the transducer was itself placed at the surface. An image 82 ofthe transducer 82 is shown in FIG. 7 which illustrates that an undesiredconical beam wave front 88' does not spread so much over the surface 86as to affect the entire area detector. The distance between A and B onthe liquid surface 86 will be free of this edge effect and may thenreceive an object modified beam for interference with the plane wavereference beam 80 to form a standing wave hologram with low noise.

Referring to FIG. 8, the technique of imaging a transducer 26 onto aliquid surface 90 is shown. All elements of FIG. 8 which are the same asthose illustrated with respect to FIG. 5 are given the same referencenumerals, the primary difference between these two figures being theexistence of a lens 92 placed between the transducer 26 and the liquidsurface. The focal length of the lens 92 as well as its placement withrespect to the transducer 26 and the surface 90 are chosen to image thetransducer into the surface. Furthermore, the image of thetransducer-226 may be made larger than the transducer itself by choosingthe focal length and distances appropriately. Being able to choose thesize of the transducer image then allows the area of the beam 94 whichstrikes the surface 90 that is free of the undesired transducer edgeeffects as well as allowing control of the energy density striking thesurface for a given transducer. The ability to make the image largerthan the transducer itself allows use of a smaller transducer than wouldbe possible without use of the lens 92, thus effecting a substantialcost saving in carrying out this invention.

Another technique for eliminating the undesirable edge effects of thetransducer is to place a pinhole stop 96 at the focal plane of a lens98, as illustrated in FIG. 9. A transducer 100 will emit a plane wave102 including the undesirable spherical wave front produced by the edgesof the transducer. The lens 98 will bring to focus at a pinhole 104 onlythe plane wave component of the beam 102. The pinhole stop 96 will thenblock any irregular component of the wave front such as illustrated by aray 106. The result is a beam of energy 108 with a very regularspherical wave front. The pinhole stop 96 should preferably be formed bya sheet of sound absorbing material such as a suitable synthetic rubber.The preferred diameter p of the pinhole 104 is approximately theresolution capability of the lens. as given by the following expression:

p=2.44fl(fld) (3) where (t represents the wavelength of ultrasound inwater that is generated by the transducer 100, f represents the focallength of the ultrasonic lens 98 and d represents the effective diameterof the lens 98.

It should be noted that the configuration illustrated in FIG. 9 hasgreat flexibility since the distance between the transducer 100 and thelens 98 is determined only by convenience. Furthermore, the entireconfiguration may be located with respect to an area detector atdistance to give coverage of the detector by the beam 108 and todetermine the density of that beam where it strikes the detector.Besides producing a spherical beam 108 which eliminates the edge effectsof the transducer 100, this configuration which uses a pinhole stop 96eliminates any other distortion in the wave front 102 which may be dueto other imperfections of the transducer 100, such as a nonflat surface.

The techniques of this aspect of the invention may also be applied toimprove the wave front of other than the flat quartz transducers so farillustrated. Referring to FIG. 10, a spherically shaped piezoelectrictransducer 110 with a virtual point source of ultrasonic energy-112 isshown. Such transducers often do not emit a'perfect regular sphericalwave front 114 because of some irregularity in the shape of the surfaceof the transducer. However, by using a lens 116 placed to image thevirtual point source 112 to a point at a pinhole 118 in a pinhole stop120, a regular spherical wave front 122 will be formed. A furtheradvantage of using a pinhole stop is to make selection of a transducermuch less critical and perhaps allowing use of transducers which areless expensive to manufacture.

Instead of a spherical shaped transducer 110 as illustrated in FIG. 10,a flat transducer with some lens element attached could be substitutedtherefor. The lens element would be one to convert the plane waveemitted from a flat transducer into a spherical wave front. Thistechnique, in combination with that illustrated in FIG. 10, allows aflat transducer of very small area to be used and still provides adesirable regular spherical wave front for use as a reference beam.

A preferred structure and method of construction of an ultrasonic lenswhich may be utilized where a lens is shown in other portions of thisdisclosure will be described with reference to FIGS. 11 through 16A.

ln most ultrasonic testing applications, the energy transmitting mediumis a liquid because of high propagation efficiency and becauseultrasound may be passed through a solid object under test immersed inthis liquid without the high energy reflections that are encountered ifair or other gas is used as the ultrasonic transmitting medium. Becauseof its compatibility with objects being tested, water is generally usedas the liquid medium. it is in this environment, then, that a lens forimaging an ultrasonic wave front at one plane onto another plane withinthe liquid medium is desired without significant aberrations or energyloss. Lenses of a solid material, such as metal or plastic, have beensuggested and used in certain applications', but'the energy losses,especially by reflection at the interfaces between the lens material andwater, are so large as to require a high energy level of the generatedultrasonic beam. A high energy level is undesirable in many applicationsbecause of possible damage to an object under investigation, and fortechnical reasons. Furthermore, positive solid lenses, such as plasticused in a water medium often have too long a focal length or too high anegative radius of curvature. Therefore, lenses have long been soughtthat do not cause such a large energy loss but yet have good imagingqualities.

ln any lens, an index of refraction n may be defined as the velocity ofthe ultrasound in the surrounding medium, herein considered to be water,divided by the velocity of ultrasound in the lens material, which may beexpressed as:

Ultrasound is refracted by such a lens and much of the analysis used inoptics applies to ultrasonic imaging, including the following equationfor determining the focal length f of a biconvex lens having surfaceswith radius of curvature R:

1 2(n-1) f R Therefore, from equation (5) it is seen that the focallength of the lens may be controlled by changing its index of refractionn or its radius of curvature R.

Ultrasonic energy loss by reflection is one significant problem indesigning an ultrasonic lens but can be made to be near zero if thefollowing expression is satisfied;

pm fip. 1 where p is the density of water and p is the density of thelens material. Solid materials popularly used for lenses do not satisfythis requirement and therefore have an energy loss by reflection of from12 to 60 percent, as more fully described in the aforementioned bookSonics, page 265.

This high reflection loss characteristic of solid lenses has beensignificantly reduced by using a liquid lens in a configuration such asthat shown in FIG. 11, wherein an available liquid 124 having a pvproduct very nearly equal to that of water or other-surrounding liquidmedium is enclosed in two spherical shells 125 and 126. Such lenses havebeen made with the liquid 124 being carbon tetrachloride and theenclosing found that shells made of metal material are not preferredbecause their pv product is not matched to that of water and thusrequires a very thin material in relation to the wavelength to keep thelosses within a tolerable level. For high frequencies, such as thosewithin the range of l to 10 MHZ. there is a very small wavelength sothat a metal shell having only small losses must be so thin it is notstructurally sound.

A restriction imposed by the use of metal shell material at anywavelength is that it must be formed with a particular radius ofcurvature for a desired focal length which cannot be altered. It hasbeen suggested to make an ultrasonic lens using a flexible material suchas rubber, for the shells 125 and 126, which are held firmly against aninner supporting ring 129 by outer supporting rings 130 and 131. Theliquid material 124 is then introduced through an opening 128 to expandthe rubber shells 125 and 126 into a desired focal length. Thisincorporates the variable focal length feature which is desired in manyapplications. However, this rubber material is not satisfactory becauseit cannot be made thin enough to reduce energy losses and still maintaina desired shape, especially at the: higher ultrasonic frequencies. Ithas been discovered that if the shells 125 and 126 are made from thinpolymeric membranes, the shells may be made very thin relative to theultrasonic wavelength used and still maintain strength and resistance tocreep over time. Also, most polymeric materials have a favorable pvproduct in comparison with water, which means that the membranes neednot be made so thin, as compared to metal, in order to obtain the samedegree of transmission efficiency to ultrasound. Furthermore, polymericfilms will stretch, which allows making a lens with a variable focallength depending on the volume of liquid material 124 placed in thecavity between the shells 125 and 126.

As with optical lenses, the ultrasonic lens of FIG. 11 will havespherical aberrations that are undesirable. These can be minimized,however, by making the radius of curvature of the shells 125 and 126very large with respect to the effective diameter d of the lens, therebyto produce a thin lens. It has in excess of L in order to produce a lenswith sufficiently short focal lengths. The problems of a high negativecurvature required for solid lenses of metal or plastic do not existwith a liquid filled lens because liquids are available with a highindex of refraction. A preferred liquid material 124 has been found tobe trichloro-trifluoro-ethane which has an index of refraction n=2.07and a good v match for low reflection losses when the lens is used inwater at room temperature. Other halogenated hydrocarbon compounds thathave been found to be useful for the sound refracting fluid of this lensare as fol lows: Carbon tetrachloride, chloroform, ethyl bromide, ethyliodide, methyl bromide, and methyl iodide.

It has been discovered that if the shells 125 and 126 made of apolymeric membrane are merely attached to the ring mem bers 129, 130 andI31, the introduction of a liquid 124 between the membranes will causethem to expand but not to form a satisfactory spherical surface. Asshown in FIG. 12, the membranes 133 and 134 become baggy because theliquid 135, being much denser than water or other surrounding medium inorder to maintain low losses, produces a pressure along the bottom ofthe lens as shown in exaggerated form. Ifa gas filled lens is used inair, this problem is not presented. The seriousness of the problem withliquids can be emphasized by pointing out that deviation of themembranes H33 and 134 from that of a spherical surface of only about onewavelength will cause serious distortions in the images obtained withthis lens. This may be solved by using the lens in a horizontal positionwhere the membranes 133 and 134 would maintain the spherical shape,although perhaps not of the radii desired, but this is an undesirablelimitation to place on the use of an ultrasonic lens, especially in theapplications discussed hereinafter. This bagging effect may also beremedied somewhat by expanding the volume of fluid 135 and thusstretching the membranes 133 and 134 sufficiently so that they form agood spherical surface. However, a lens is then formed that has severespherical aberrations because of a small radius of curvature relative tothe lens diameter. It has been discovered that if the membranes 133 and136 are stretched before they are mounted into the supporting rings, athin lens may be constructed without the bagging efi'ect shown in FIG.12 and thus will produce images of improved quality.

With reference to FIGS. 13, 13A and 138, a method for prestretching thelens membranes is illustrated. A thin sheet of polymeric material 136(or other material satisfying the requirements hereinabove discussed) isclamped between two rigid metal rings 137 and 138 along with a gasket140. The inside diameter of the ring should be significantly larger thanthe diameter of the lens to be made, as will be apparent hereinafter.The rings 137 and 138 are clamped firmly together by several screws Mil.This assembly is then placed over a ring 142 and fastened tightlythereto by screws 143, thereby stretching the polymeric film 136. Acenter supporting ring 144 and an outer supporting ring 145 are thenclamped on either side of the polymeric film 136 to form a portion ofthe lens structures shown in cross section in FIG. 14A. These two ringsare held tightly together by several screws 1 37. A second polymericfilm 148 may then be stretched in the same manner as shown with respectto FIGS. 13, 13A and 13B and clamped between the center support ring 144and an outer support ring 149 by the use of screws 150.

FIG. 14 shows the completed lens in plan view. A needle valve 152 isthreadedly attached to the interior of an opening 153 of the centersupport ring 144 to control the flow of fluid into the cavity formedbetween the polymeric films 136 and 148. As fluid is fed through thisvalve into the cavity, air may be let out by opening a bleeder valve 154which is threadedly attached to the central support member in an opening155. When all the air has been exhausted from the cavity, the bleedervalve 154 is closed and fluid is forced through the needle valve 152until the polymeric membranes 136 and 148 take on a spherical shape witha desired radius of curvature.

FIG. 15 illustrates a plunger which may be used for controlling theamount of fluid placed into the lens. A chamber is formed by housing 181which includes a plunger 182 having O-rings I83 and 184 to form a seal.A shaft 185 threadedly attached to an end of the housing 181 providesreciprocal power to the plunger 182 to force liquid out of the chamber180 through the tube opening 186 of the tube 188. This plunger assemblymay be permanently connected through tube 188 to the needle valveassembly 152 to allow control of the amount of fluid in the lens andthereby its focal length.

In constructing a lens with this novel prestretching step, the materialchosen for the membranes should have a high yield point and should beprestretched so that when in the final lens and stretched further byfluid pressure from within, the stress within the membrane will be closeto its proportional limit. This will improve conformity to a sphericalshape.

Several polymeric films are available commercially which are suitablefor the membranes. Typical polymeric films include: polyesters (e.g.,polyethylene terephthalate), polyimides (e.g., the polyimide ofpyromellitic acid and bis (4- aminophenyl), ether), fluorocarbonpolymers and copolymers (e.g., the copolymer of tetrafluoroethylene andhexafluoropropylene), polyvinyl fluoride, polyvinylidene chloride,polyolefins, and the like. The polymeric film may be heat shrinkable ifdesired, such as heat-shrinkable polyethylene terephthalate film. Also,acetate film may be made of cellulose acetate which is normally preparedby solvent casting without orientation.

Although the improvement has been described to be a double-concave lens,it should be understood that many variations are possible in the form ofthe lens while still taking advantage of the improved combination ofmaterials and methods ofconstructing the lens of this invention. Forinstance, a double-concave lens could be constructed, as outlined above,except that when all the air is driven from the cavity between the twomembranes I36 and 148 of FIG. 14A, fluid would be withdrawn. Thesurrounding water or other ultrasonic transmitting medium will supplythe necessary pressure for the membranes to take on concave shapesdependent upon the volume of liquid remaining therebetween. Furthermore,a lens may be constructed according to the improvements of thisinvention with only one membrane. The other member enclosing the lensfluid may be a solid element or even a transducer.

A membrane may also be placed under stress after lens manufacture by aheat-shrinking technique. The desired volume of lens fluid is placed ina lens cavity defined by shells of heat-shrinkable polymeric films.Heating the films develops stress required for a spherical lens shape.

A further technique for stretching membranes according to this inventionis shown in FIG. 16. Membranes 189 and 19b form a cavity 191therebetween along with the middle support ring 192 and gaskets I93 and194. The membranes 189 and are held in ring assemblies 195 and 196similar to that shown in FIG. 13, and are attached through a threadedmember 197 to the center ring support 192. The center ring support 192and the membranes 189 and 190 are placed loosely together in fonningthis lens and then a specified volume of liquid is introduced into thecavity 191. Wingnuts 198 and 199 are then turned to stretch themembranes until a lens is formed having spherical surfaces. Additionalring clamps 156 and 157 having gaskets 158 and 159 are added to improvethe seal of the volume 191. This embodiment of the invention also allowsfurther stretching of the membranes after the lens has been formed tocompensate for creep.

A further embodiment of this invention is shown in FIG. 16A where alens-refracting liquid 362 is placed between two membranes 350 and 352which have been prestretched as hereinabove described. A volume 354 isformed by adding a third membrane 356. A third chamber 358 is formed byadding a fourth membrane 360. The chambers 3 54 and 358 are filled withthe same liquid in which the lens is immersed, which will most often bewater, so there is no refracting of be a lens with the same imagingproperties as those discussed hereinabove, but will have the advantagethat the membrane 350 and 352 will be less likely to creep over a periodof time. Furthermore, if the pressures in chambers 354 and 358 are madeunequal, the membranes 350 and 352 will take on a radii of curvaturethat are not equal, therefore giving possibility to make a lens withfurther correction for spherical aberration.

In the altemative, the configuration of FIG. 16A could be used for amultielement lens for aberration correction. Soundrefracting liquid maybe inserted in the chambers 354 and 358 as well as or in place of withinthe chamber defined by the membranes 350 and 352. The liquids placed inthese three chambers may have different or similar refractory propertiesdepending upon the desired result. Furthermore, one or more of themembranes may be made to take different directions of curvature than thedirections shown in FIG. 16A by adjusting the relative volumes of liquidin the three chambers.

Throughout this description, it has been assumed that a lens will bemade that is spherical but it should be understood that the techniquesof this invention have equal applicability to a cylindrical lens or oneof some other shape that such a stressed membrane will tend to assumewhen its edge is held in a particular manner and filled with a desiredvolume of liquid.

An improved technique will now be described for using ultrasound toimage the internal structure of a woman's breast in order to detect agrowth therein, such as abnormal cancer tissue, by use of eithertwo-dimensional direct imaging or threedimensional holographic imaging.Breast testing has generally been accomplished by X-ray techniques whichhave certain limitations because of a possible health hazard and becausethe resulting picture of the breast only indicates density variationwhich gives a limited amount of information. Pulse echo ultrasonicmethods have been used for imaging the female breast, as mentionedhereinabove, but this has certain limitations since a real time view ofthe entire breast in one picture is not possible. I

In the ultrasonic technique described herein, a beam of ul trasonicenergy penetrates the entire breast and results in an optical image ofthe ultrasonic field passing through the breast. This image indicatesmore than mere density variations of the breast; it also showsdifferences in molecular binding as well. This technique allowsdetection of cancer or other ab normal internal growths in earlierstages of growth than do other methods. Although three-dimensionalholographic imaging of the female breast is preferred, the invention mayalso be practiced to advantage with two-dimensional direct imagingmethods.

Since ultrasound is heavily attenuated in a gas such as air and isfurther heavily reflected when passing from air to a solid object, afemale breast is best imaged by submersing it in an ultrasonictransmitting liquid medium of high efficiency. This medium should becompatible with the female breast so that no more than a smallproportion of energy will be reflected as the ultrasound passes from theliquid medium to the breast and through to the liquid medium again. Ithas been found that water is quite satisfactory and, of course,available in large quantities and is further comfortable for thepatient. To avoid having to submerse more of the patient in the waterthan the breast under examination, it has been found preferable toposition the woman in a horizontal position and extend the breast underexamination down through a top surface of the water and pass theultrasonic energy beam substantially parallel to the water surfacethrough the breast. FIG. 17 illustrates this preferred technique. A tank200 contains an ultrasonic transmitting water 202 and a woman 204 isheld in a prone position over the tank by a supporting surface 206. Abreast 207 under examination is suspended into the water 202 through anopening in the supporting surface 206. A quartz transducer 208 held inplace We housing 210 is a preferred.

source of a substantially plane wave ultrasonic beam 212 which ispropagated through the water 202 to the breast 207.

The ultrasonic wave front passing through the breast is imaged by anultrasonic lens 213 onto an ultrasonic detector such as the areadetector 214 which is shown to be of a type such as a Pohlman cell. Alight source 216 illuminates the observer's side of the area detector214, thereby displaying in the optical domain a representation of theultrasonic wave front which has passed out of the breast 207. It shouldbe understood that the area detector 214 may be any other known devicefor con verting' ultrasonic wave front variations into the opticaldomain, such as the ultrasonic camera hereinbefore described.

A primary consideration in the configuration shown in FIG. 17 is toallow room for the transducer housing 210 above a surface 218 of waterfilled container 200 which then permits the ultrasonic energy beam 212to be propagated very close to the surface 218 and thus image a largeportion of the breast 207 without having to immerse any more of thewoman into the water. To allow as much of the breast 207 as possible tobe placed into the liquid 202, the supporting surface 206 has beensloped from both directions to the breast opening.

It has been discovered that the best imaging is obtained if the breast207 is held down away from the chest cavity of the patient so that asmuch of the breast is in the path of the ultrasonic beam 212 aspossible. It is also desirable that the breast is flattened to minimizethe maximum thickness and held immobilized during the examination. Thishas been found to be satisfactorily accomplished by a breast holderwhich includes two supporting plates 220 and 222 which serve to keep thebreast 207 under gentle compression during the examination. Asmay bebetter seen by reference to FIG. 18, the plates 220 and 222 each supportthin films 224 and 226 which are substantially transparent to theparticular ultrasonic wavelength being used. These sheets 224 and 226are preferably a suitable polymeric film and should be stretched to beheld under tension. The frame 222 is rigidly attached to thepatient-supporting member 206 and the frame 220 is attached to aseparate plate 228 which is held in a slideable relationship with theother components so that it may be adjusted on each individual patient.The plates 220 and 222 are preferably at an angle with the supportingmember to compress the breast more in'the portion near the chest cavitythan in the remaining portions of the breast in order to hold it outaway from the chest cavity as much as possible. However, this angleshould not be so great as to cause the breast to have a considerablyuneven thickness across the ultrasonic beam 212 and it has been foundthat an angle somewhere around 10 from the perpendicular with thesupporting member 206 is preferred, as shown in FIG. 18.

A preferred range of frequencies of the ultrasonicenergy beam 212 hasbeen found to be between 1 and 10 MHz. Below 1 MI-lz., resolution hasbeen found to be inferior, and above 10 MHz. a breast is not transparentto the ultrasonic energy. An operating frequency of 3 MHz. has beenfound to be preferred for breast examination. The transducer 208 ispreferably an X-cut quartz crystal with a l M Hz. fundamental frequencywhich is operated at 3 MHz. by a power supply at that frequency,according to well-known techniques. The power supply equipment could behoused within the examination equipment next to the tank 200 in an area230 of FIG. 17.

The technique of this invention as described with respect to FIGS. 17and 18 provides for real time examination of a breast. This has theadvantage that the breast may be properly positioned and flattened andother parts of the apparatus adjusted, such as the ultrasonic lens 213,in order to give a good image that is of value for medical diagnosticwork.

The preferred technique for ultrasonic breast imaging utilizes thetechniques of wave front reconstruction of holography, as hereinabovedescribed. Apparatus for carrying out the holographic technique isillustrated, particular in schematic form, in FIGS. 19, and 21. Sincethese Figures are different views of the same apparatus, commonreference numbers have been used for all views.

An object beam quartz transducer 232 supported by a holder 234 generatesan object beam of ultrasonic energy 236 which passes through a breast238 positioned between two breast-holding plates 240 and 242. The wavefront leaving the breast is imaged by an ultrasonic lens 244 onto anisolation tank 246 which is the area detector. A thin membrane 248 whichis transparent to the ultrasonic frequency used, and is preferably apolymeric film, separates the water 250 from the tank of water 252 butallows ultrasonic energy to travel therebetween without significantattenuation. A mirror 254 capable of reflecting an ultrasonic beam ofthe frequency being used without significant attenuation is placed todirect the horizontal traveling ultrasonic object bearing beam 256 intothe horizontal isolation tank 246. A glass sheet 0.5 inch thick has beenfound satisfactory for the mirror 254.

A reference beam 258 of ultrasonic energy at a frequency substantiallythat of the object beam 236 is generated by a quartz transducer 260supported in a holder 262. The transducer 260 is imaged by an ultrasoniclens 264 onto the isolation tank 246. A thin stretched membrane 266 isprovided to pass the reference beam between the two tanks of liquid 250and 252. A mirror 268is positioned to reflect the horizontally travelingbeam into the isolation tank 246. An ultrasonic frequency ofapproximately 3 MHz. is preferred for both the object and referencebeams and may be generated, if desired, by driving the transducers 232and 260 by a common power supply.

The object bearing beam 256 and the reference beam 258 interfere witheach other to produce standing waves in a liquid surface of theisolation tank 246. The standing wave pattern is capable of diffractinglight into various image-carrying orders. For best results, it has beenfound that the beams 256 and 258 should interfere at an angle ofapproximately 60 with one another when the frequency utilized is 3 MHz.For difi erent frequencies and other varying circumstances, thereflectors 254 and 268 may be changed in orientation relative to oneanother along with moving the isolation tank 246 either up or downrelative to the reflectors.

The area detector is a thin film of liquid 270 held within the isolationtank 246 by a round frame 272 and is separated from the water 252 by athin membrane 274 stretched across the frame 272. The thin liquid film270 is preferably one having a viscositysubstantially greater than thatof water so that the standing wave pattern formed on its surface willnot be disturbed by minor mechanical vibrations, fluorocarbon beingpreferred. The thin stretched membrane 274 is preferably made of alow-gloss black polyvinyl chloride film which is transparent toultrasound.

The standing wave pattern formed on the surface of the thin liquid film270 as a result of the interference of the object bearing beam 256 andthe reference beam 258 may be optically read as illustrated in FIG. 21.A substantially monochromatic light source 276 of small diameter iscontrolled as to beam spread by a lens 278 to illuminate substantiallyall of the standing waves formed on the liquid surface 270. This lightis then diffracted and its various orders are separated by a lens 280 sothat a spatial filter 282 may be placed where these various orders ofdiffracted light come to a point focus and filter all but one firstorder beam 284. A lens 286 may be desired to control this light which isdirected to a mirror 288, through another beam-controlling lens 290 toanother mirror 292 to provide a three-dimensional breast-image-carryingwave front in a convenient location for real time observation. In thealternative, the mirror 288 may be pivoted about an end 292 into alocation to reflect the first order diffracted light beam 284 into acamera 294 to make a photograph of the image of the internal breaststructure. In either case, the viewing optics are focused on the portionof the three-dimensional image desired to be viewed. If the viewingoptics be focused on a plane passing through the image which iscoincident with the liquid surface 270, the size and wavelengthrequirements of the light source 276 are reduced as hereinbeforediscussed in detail.

If a permanent hologram is desired so that a three-dimensional image ofthe internal breast structure can be reconstructed at some future date,an image-carrying first order diffracted beam 284 (the light source 276is now made to be a coherent one) is interfered with an off-axisreference beam coherent with the source 276 and a photographic plate isexposed to this interference pattern. When developed, the photographicfilm is a permanent hologram which allows an image of the breaststructure to be reconstructed by shining a substantially monochromaticlight through the film. As an alternative to using a separate referencebeam to make a permanent hologram, a second first order diffracted beam296 could be caused to interfere with the first order diffracted beam284 with the interference pattern recorded on film, but the results arenot preferred in many circumstances.

Referring to FIGS. 22 and 23, a preferred patient support surface andbreast-holding mechanism, respectively, are illustrated for use witheither two-dimensional direct imaging or three-dimensional holographicimaging. A patient support surface 300 of FIG. 22 has a depressedsurface area 302 for allowing the patient's chest cavity to be placedclose to the liquid medium below and her breast placed in the liquidthrough an opening 306. A further depression 304 in the depressedsurface 302 is designed for the patients right shoulder so that she maylie slightly on her right side and thereby cause her rib cage to contactthe edges of the opening 306 for most of the breast's' perimeter. Thisfirm contact is necessary to prevent the breast from slipping up and outof the holder below. In a similar manner, another surface depression 308is provided for the patients left shoulder to facilitate placing herleft breast as far through the opening 306 as is possible withoutslipping out ofthe holder.

Surrounding the opening 306 on the underneath of the support surface 300is a breast holder of preferred construction which can be seen in detailby reference to FIG. 23 and which is claimed in a copending applicationby Henry S. Jones, filed simultaneously with the present application. Aboxlike frame 310 has as one end thereof a U-shaped support plate 312which holds a thin membrane 314 in tension. This membrane should besubstantially transparent to the ultrasonic frequency being used. Themembrane 314 is preferably a polymeric film such as a polyester,polyethylene or polypropylene. A second U-shaped support plate 316 holdsa similar membrane 318 in a stressed condition. The support plate 316 isplaced within the frame 310 to form a breast cavity between themembranes 314 and 318. The support plate 316 further has cylindricalguide members 320 and 322 attached to opposite sides thereof. Thesecylindrical members are mounted within guide slots 324 and 326 of theframe 310 in a manner so that the support plate 316 may be rotated aboutthe centerline common to the cylindrical members 320 and 322 and furtherto be slideable along the two guide slots 324 and 326.

The support plate 316 is resiliently held away from the support plate312 by a spring 327 connected between the cylindrical member 320 and theunderside of the support surface 300, and a second spring (not shown)between the cylindrical member 322 and the support surface 300. When abreast is positioned between the membranes 314 and 318, the supportplate 316 is drawn toward the support plate 312, thereby placing thebreast under compression. This movement is made by operating a controlknob 332 which is operably connected through a control gear box 335 withthe upper side of the U- shaped plate 316 by means of ropes 328 and 330.Similarly, motion from the turning of the control knob 334 iscommunicated to the bottom edge of the U-shaped plate 316 by ropes 331and 333 through the control gear box 335. Several pulleys are shown overwhich these four ropes move between the support plate 316 and the gearcontrol box 335.

To operate the breast holder, movement is applied to the ropes 328 and330 initially to draw the top edge of the membrane 318 against thebreast, thereby to hold the breast away

1. A method of producing images from an ultrasonic hologram which displays an interference pattern resulting from the intersection of two mutually coherent ultrasonic energy beams at a finite angle with each other, one of said beams being modified by an object, comprising the steps of: illuminating the ultrasonic hologram with light radiation to produce various diffracted orders of said radiation including at least one object-image-carrying order, and simultaneously viewing in said at least one object-imagecarrying order an image of the object by focusing viewing optics upon the ultrasonic hologram.
 2. A method of rendering an image of an object under investigation, comprising the steps of: directing a first beam of ultrasonic energy toward the object and thence to a hologram detecting surface responsive to an uLtrasonic energy interference pattern in a manner that light is diffracted thereby, simultaneously directing a second beam of ultrasonic energy to the hologram detecting surface to intersect the first beam at a finite angle therewith, said first and second beams being mutually coherent, illuminating the hologram surface with light radiation in a manner to diffract the radiation into various diffracted orders, and viewing an image of the object in one of the diffracted orders by focusing viewing optics upon the hologram detecting surface.
 3. The method according to claim 2 wherein the step of directing a first beam additionally includes imaging an ultrasonic field that exists at the object as a result of the first beam striking the object onto the hologram detecting surface.
 4. Apparatus for producing an image from an ultrasonic hologram which displays an interference pattern resulting from the intersection of two mutually coherent ultrasonic energy beams at a finite angle with each other, one of said beams being modified by an object, comprising: means for illuminating said ultrasonic hologram with light radiation whereby said radiation is diffracted by said ultrasonic hologram into various diffracted orders including at least one object-image-carrying order, and optical viewing means positioned in the path of said at least one object-image-carrying order and focused upon said ultrasonic hologram for viewing an image of said object carried thereby.
 5. Apparatus for producing an image of an object under investigation, comprising: means for producing an ultrasonic hologram at an area detecting surface characterized by responding to an ultrasonic energy interference pattern in a manner that light is diffracted thereby, said last-named means including, means for directing a first ultrasonic beam to said object and thence to said detecting surface, and means for directing a second ultrasonic beam to said detecting surface to intersect said first beam at a finite angle therewith, thereby producing an ultrasonic hologram, said first and second beams being mutually coherent, means for illuminating said ultrasonic hologram with light radiation in a manner to diffract said radiation into various diffracted orders, and viewing optics positioned in a first diffracted order of said beam and focused upon said ultrasonic hologram.
 6. Apparatus according to claim 5 and including an ultrasonic lens positioned in the path of said first ultrasonic beam between said object and said detecting surface for imaging an ultrasonic field which exists at said object onto said detecting surface.
 7. In a method of holographic imaging including the steps of directing an object beam of ultrasonic energy toward an object and thence to an area detector responsive to ultrasonic energy and simultaneously directing a reference beam to the area detector for interference with the object beam to produce an interference pattern on the area detector, said object and reference beams being mutually coherent and intersecting at a finite angle with each other, the improvement comprising the step of: imaging the ultrasonic field at the object onto the area detector.
 8. A method according to claim 7 wherein the imaging step includes placing a lens between said object and said area detector.
 9. In a method of holographic imaging including the steps of directing an object beam of ultrasonic energy toward an object and thence to an area detector responsive to ultrasonic energy and simultaneously positioning an ultrasonic generator to direct a reference beam of ultrasonic energy mutually coherent with the object beam to the area detector to intersect the object beam at a finite angle to produce an interference pattern on the area detector, the improvement comprising the step of: imaging the ultrasonic field at the reference beam generator onto the area detector.
 10. A method according to claim 9 wherein the imaging step includes placing a lens between the ultrasonic generator and the area detector.
 11. The improved method according to claim 9 which includes the additional step of imaging the ultrasonic field at the object onto the area detector.
 12. In a method of holographic imaging including the steps of directing an object beam of ultrasonic energy toward an object and thence to an area detector responsive to ultrasonic energy and simultaneously positioning an ultrasonic generator to direct a reference beam of ultrasonic energy mutually coherent with the object beam to the area detector to intersect the object beam as a finite angle to produce an interference pattern on the area detector, the improvement comprising the steps of: placing an ultrasonic lens between the ultrasonic generator and the area detector, and positioning a pinhole filter relative to the lens so that a point image of the generator is passed through the filter, thereby resulting in a regular spherical reference beam wave front striking the area detector.
 13. The improved method according to claim 12 which includes the further step of placing an ultrasonic lens between the object and the area detector to image the ultrasonic field at some surface within the object onto the area detector.
 14. Apparatus for producing an ultrasonic hologram, comprising, means for generating and directing ultrasonic energy at an object and thence as object modified energy to an area detector responsive to ultrasonic energy, means for generating reference ultrasonic energy mutually coherent with the object-illuminating ultrasonic energy and for directing said reference energy to intersect the object modified ultrasonic energy at a finite angle therewith at the area detector, and means for imaging said reference-ultrasonic-energy-generating means onto said area detector.
 15. The apparatus of claim 14 wherein said reference-ultrasonic-energy-generating means includes a substantially flat transducer and wherein imaging means includes an ultrasonic lens placed between said generating means and said area detector.
 16. Apparatus for producing an ultrasonic hologram, comprising: means for generating ultrasonic energy directed at an object and thence object modified energy to an area detector responsive to ultrasonic energy, a reference-ultrasonic-energy-generating transducer placed to direct the ultrasonic energy to intersect the object modified ultrasonic energy at a finite angle therewith at the area detector, said object illuminating and reference ultrasonic energy being mutually coherent, an ultrasonic lens placed between said reference energy transducer and said area detector, and a pinhole filter placed between said lens and said area detector.
 17. Apparatus according to claim 16 wherein said reference-ultrasonic-energy-generating transducer includes a substantially flat quartz transducer and wherein said pinhole filter is placed in the focal plane of said lens.
 18. Apparatus according to claim 16 wherein said pinhole filter is placed so that an apparent point source of said transducer is imaged at its pinhole. 